An information processing device employed in an information processing system that must be highly safe and reliable needs a periodical diagnosis of a storage device. Since the storage device needs to be diagnosed for general memory failures and also memory soft errors attributable to radioactive particles, the diagnosis of the storage device has to be performed not only while the system is in the process of booting-up but also while the system is in operation.
In general, diagnosis of a memory is performed by a diagnosis method with a marching pattern, for example, more specifically, by firstly writing a predetermined value in a particular address of the memory and then by comparing the value read from the same address with the predetermined value thus written to check whether the two values are the same. Such methods make diagnoses by rewriting values stored in the memory. Accordingly, no diagnosis can be performed on the memory regions that are being accessed by the CPU.
In addition, an information processing devices such as a CPU generally uses storage devices in a hierarchical structure (the structure is called a storage hierarchy) including a fast, small-capacity memory (referred to as a cache memory) such as a SRAM that is provided between the information processing device and the main memory and is used for avoiding delays of the operation of slow, larger-capacity main memory such as a DRAM when acquiring or updating information such as programs or data.
The CPU usually accesses only the cache memory. There are two write control methods: the write-through method in which write accesses to the main memory and the cache memory are made simultaneously; and the write-back method in which the batch writing is performed on the main memory to minimize the write accesses to the main memory.
In some of storage devices equipped with cache memories, the cache memories block the direct access to the main memories. In this case, diagnosis of the main memories requires a special testing apparatus.